This invention relates to a vehicle suspension control system. By "vehicle" is meant all classes of vehicle capable of motion on land, and the term includes motor vehicles, lorry cabs and trailers, tractors and tracked vehicles.
In particular, the invention relates to a vehicle suspension control system for controlling the suspension of a vehicle having an active suspension system.
Active suspension systems are well known. For example, our European Patent Application No. EP-A-0114757 discloses an active suspension system in which a number of sensor inputs representing variables of vehicle motion are processed in a microprocessor to produce signals representing forces required at the points of support of the vehicle body on its wheel/hub assemblies. The required forces are applied by respective hydraulic actuators disposed between the vehicle body and each wheel/hub assembly to maintain as nearly as possible a constant attitude of the vehicle.
Active suspension systems operate on the principles either of measuring the forces acting between the vehicle body and the wheel/hub assemblies or of calculating such forces from measured values indicative of vehicle behaviour.
The main problem faced by the designer of an active suspension system is the difficulty that whilst the vehicle suspension system should respond to perturbations in the road (dynamic inputs) by moving the actuators attached to the wheel and hub assembly, the suspension system should not cause the actuators to deflect in response to loads imposed upon the vehicle by reason of "steady state inputs". "Steady state inputs" include loads being placed on the suspension due to the cornering and acceleration/deacceleration of the vehicle. "Steady state input" also include variations on the load of the vehicle due to load carried inside the vehicle and aerodynamic forces on the vehicle. Such "steady state inputs" should generally not cause displacement of the actuators. In other words, the designer is faced with the problem of designing a suspension system which is "soft" to road input whilst being "hard" to steady state inputs.